Search

To better understand the differences in the efficacy of entomopathogenic nematode species against white grub species, we are studying the various steps of the infection process of entomopathogenic nematodes into different white grub species using nematode species/strains with particular promise as white grub control agents. In this study we compared the attraction of the entomopathogenic nematodes Steinernema scarabaei (AMK001 strain), Steinernema glaseri (NC1 strain), Heterorhabditis zealandica (X1 strain), and Heterorhabditis bacteriophora (GPS11 strain) to third-instars of the scarabs Popillia japonica, Anomala orientalis, Cyclocephala borealis, and Rhizotrogus majalis, and late-instar greater wax moth, Galleria mellonella, larvae. Individual larvae were confined at the bottom of 5.5 cm vertical sand columns, nematodes added to the sand surface after 24 h, and nematodes extracted after another 24 h. Nematode attraction to hosts was strongly affected by nematode species but the effect of insect species varied with nematode species. S. glaseri had a high innate dispersal rate (i.e., in absence of insects) and was strongly attracted to insects without significant differences among insect species. S. scarabaei had a very low innate dispersal rate so that even a strong relative response to insects resulted in low absolute dispersal rates toward insects. S. scarabaei tended to be most attracted to G. mellonella and least attracted to C. borealis. H. zealandica had a high innate dispersal rate but only responded weakly to insects without significant differences among species. H. bacteriophora had limited innate dispersal and only weakly responded to insects with G. mellonella tending to be the most attractive and C. borealis the least attractive insect. It has to be noted that we cannot exclude that the use of different rearing hosts (A. orientalis and P. japonica larvae for S. scarabaei, G. mellonella larvae for the other nematodes) might have had an impact on the nematodes dispersal and relative attraction behavior. This study indicates that host attractiveness and nematode dispersal rates may contribute but do not play a major role in the variability in white grub susceptibility and/or nematode virulence.

We tested the effect of soil type on the performance of the entomopathogenic pathogenic nematodes Steinernema scarabaei, Steinernema glaseri, Heterorhabditis zealandica, and Heterorhabditis bacteriophora. Soil types used were loamy sand, sandy loam, loam, silt loam, clay loam, acidic sand, and a highly organic potting mix. Infectivity was tested by exposing third-instar Anomala orientalis or Popillia japonica to nematodes in laboratory and greenhouse experiments and determining nematode establishment in the larvae and larval mortality. Infectivity of H. bacteriophora and H. zealandica was the highest in potting mix, did not differ among loamy sand and the loams, and was the lowest in acidic sand. Infectivity of S. glaseri was significantly lower in acidic sand than in loamy sand in a laboratory experiment but not in a greenhouse experiment, and did not differ among the other soils. Infectivity of S. scarabaei was lower in silt loam and clay loam than in loamy sand in a greenhouse experiment but not in a laboratory experiment, but was the lowest in acidic sand and potting mix. Persistence was determined in laboratory experiments by baiting nematode-inoculated soil with Galleria mellonella larvae. Persistence of both Heterorhabditis spp. and S. glaseri was the shortest in potting mix and showed no clear differences among the other substrates. Persistence of S. scarabaei was high in all substrates and its recovery declined significantly over time only in clay loam. In conclusion, generalizations on nematode performance in different soil types have to be done carefully as the effect of soil parameters including soil texture, pH, and organic matter may vary with nematode species.

This study describes the basic ecological characteristics of the entomopathogenic nematode Steinernema scarabaei (Rhabditida: Steinernematidae) that was originally isolated from epizootics in scarab populations in New Jersey turfgrass areas. Under laboratory conditions, S. scarabaei infected a limited range of insect species and appeared best adapted to scarab larvae as hosts. It uses a widely ranging foraging strategy with a low attachment rate to mobile hosts on the soil surface but with excellent infection of sedentary host placed at >or=2 cm soil depth. It has a wide thermal activity range with optimum infectivity from 17.5 to 25 degrees C. Because of its foraging strategy and adaptation to scarab larvae as hosts, S. scarabaei has outstanding potential for the control of scarab pests.

The development of five geographic isolates of Steinernema feltiae at 5, 8, 10, 15, 20, 25, and 28 degrees C in wax moth, Galleria mellonella, larvae was examined. The isolates were from Mediterranean (Sinop from Turkey, SN from France, and Monterey from California), subtropical (Rafaela from Argentina), and tropical (MG-14 from Hawaii) regions. All isolates caused 100% mortality of wax moth larvae and developed and produced progeny between 8 and 25 degrees C. At 28 degrees C, mortality was 100%, but no progeny was observed. The highest infective juvenile production was observed at 15 degrees C for all isolates. In general, the tropical isolate, MG-14, had the lowest production of infective juveniles. The time of emergence of the infective juveniles from the host cadaver showed some differences among isolates, with the Sinop isolate having the earliest emergence time from cadavers at 15 degrees C (10 days) and 20 degrees C (8 days). At 25 degrees C, the infective juveniles of the Sinop, SN, and Rafaela isolates emerged from the cadavers from 5 to 7 days. Time of host death by all isolates showed no differences at 8, 10, 15, 20, and 28 degrees C. At 25 degrees C for all isolates (except the MG-14), shorter times to host death were observed. Host death occurred at 12 days at 8 degrees C, 9 to 11 days at 10 degrees C, 4 to 5 days at 15 degrees C, 3 days at 20 degrees C, and 2 days at 25 and 28 degrees C. For penetration efficiency, the Sinop, SN, and Rafaela isolates penetrated their hosts at 5, 8, and 10 degrees C. Penetration of the infective juveniles was consistently high for all isolates at 15, 20, 25, and 28 degrees C, but it was significantly lower for the MG-14 isolate at 15, 25, and 28 degrees C. No progeny production occurred at 28 degrees C, but nematode penetration did occur with the MG-14 isolate having significantly lower penetration than the other isolates. When nematodes were produced at 8, 15, and 23 degrees C in wax moth larvae, all isolates had infective juveniles with longer body lengths at 8 degrees C followed by 15 and 23 degrees C. To further verify body length at the different temperatures, beet armyworm, Spodoptera exigua, larvae and dog-food agar medium were used, respectively, for in vivo and in vitro culture of the Sinop isolate. Infective juvenile body length showed the same trends, with the longest being at 8 degrees C and decreasing in length from 15 to 23 degrees C. The data suggest that quality of food for the nematode and temperature (that is, developmental time) influence the body length of the infective juvenile.

We determined whether nematode-trapping fungi may influence the dynamics of a coastal shrub community. The food chain interactions in the shrub community involve the dominant plant species, its major insect herbivore, and an entomopathogenic nematode, Heterorhabditis hepialus. Of the 12 nematode-trapping fungi previously isolated from soils at the study site, 5 were selected for this study. Arthrobotrys oligospora, Geniculifera paucispora, Monacrosporium eudermatum, and Monacrosporium cionopagum efficiently trapped and colonized H. hepialus on agar; in contrast Nematoctonus concurrens trapped but did not infect or colonize the nematode on agar. To determine whether these fungi can suppress H. hepialus in soil, we added the fungi in the form of fungal-colonized nematodes to pasteurized (2 hr at 62 degrees C) and raw (nontreated) soil from the study site. Suppression was measured by comparing nematode invasion into a wax moth larva in fungus-treated and untreated soil in vials at 20 degrees C. Fungal population density in soil was estimated using dilution plating and most probable number procedures. All fungi suppressed H. hepialus if the wax moth larvae were added 4 days after the nematodes. Suppression ranged between 37 and 54% and did not differ among fungi. Suppression was usually greater in raw than in pasteurized soil. Raw soil contained a constant background of nematode-trapping fungi, and A. oligospora was the most common among these; no background was detected in pasteurized soil. The presence of background fungi in raw soil may explain the higher suppression in raw than in pasteurized soil. Fungal propagule densities in our laboratory experiments were similar to those observed in the field, suggesting that nematode-trapping fungi may influence the dynamics of the plant, insect herbivore, and entomopathogenic nematode in the coastal ecosystem.

Larvae of Galleria mellonella exposed to infective juveniles of Steinernema carpocapsae and Steinernema glaseri were coinfected by both nematode species. Infective juveniles of both species that penetrated and established in their insect hosts always developed into apparently normal adults. In both nematode species, the proportion of established females was reduced in cadavers that had been exposed to both species compared with cadavers exposed to one species. Although S. glaseri and S. carpocapsae produced progeny from the same cadavers, S. glaseri was less affected by mixed infection than S. carpocapsae . In the mixed infection at a rate of 8 infective juveniles of each species, the progeny production of S. glaseri was not reduced, whereas that of S. carpocapsae was significantly reduced by 79% compared with the respective single species treatments. At a rate of 25 infective juveniles of each species, progeny production of both S. glaseri and S. carpocapsae was significantly reduced by 38 and 94%, respectively, compared with the respective single species treatments. The ratio of inocula between the two species affected the progeny production. If S. glaseri and S. carpocapsae were combined at the rates of 8:16, 16:16, and 16:18 infective juveniles, respectively, S. glaseri had reductions in progeny production of 53, 9, and 30%, respectively, whereas S. carpocapsae had reductions of 35, 85, and 98%, respectively. The intrinsic superiority of S. glaseri can be ascribed to its faster development that deprives its slower competitor of food resources. In addition, the less specific association of S. glaseri to its bacterial symbiont, Xenorhabdus poinarii , allows it to develop in cadavers colonized by Xenorhabdus nematophilus , the symbiont of S. carpocapsae .

The presence of hosts at different depths of soil and the influence of different soil moistures on the infectivities (as measured by the number of nematodes establishing in the host) of two entomopathogenic nematode species, Steinernema carpocapsae and S. glaseri , were investigated. The infective juveniles had been uniformly distributed in the soil. At a water potential of -0.006 megaPascal (MPa) in a sandy loam soil, nematode establishment at different depths was variable in both species. S. carpocapsae tended to establish at equal numbers in hosts at depths between 0 and 10 cm, whereas S. glaseri showed consistent establishment from 2 to 10 cm but was variable at 0 cm. Tests in 1-liter cups and 3-ml arenas showed that soil moisture affected nematode establishment. S. carpocapsae started establishing in hosts at -5 MPa water potential in 3-ml arenas and -0.3 MPa in 1-liter cups (low soil moistures) and had the highest establishment numbers between -0.1 and -0.01 MPa. At higher soil moistures, the number of nematodes establishing dropped significantly, especially in the 1-liter cups. The establishment range for S. glaseri was more restricted at the low soil moistures. It started establishing in hosts at -0.3 MPa in the 3.ml arenas and -0.05 MPa in the 1-liter cups, respectively. Its establishment numbers increased constantly at higher soil moistures but dropped in the 1-liter containers in water-saturated soil (-0.003 MPa). Although S. glaseri did not infect hosts after 24 hr exposure to very dry soil (-40 MPa), some S. carpocapsae still established in hosts after this exposure when the soil was rewetted to -0.006 MPa. Generally, S. carpocapsae performed better in drier conditions than S. glaseri .

We compared the penetration of the entomopathogenic nematodes Steinernema scarabaei (AMK001 strain), S. glaseri (NC1 strain), Heterorhabditis zealandica (X1 strain), and H. bacteriophora (GPS11 strain) into third-instars of the scarabs Popillia japonica, Anomala orientalis, Cyclocephala borealis, and Rhizotrogus majalis. When larvae were exposed to nematodes for 6-72 h larval mortality and nematode establishment rate and occasionally speed of kill often showed the same pattern within nematode-white grub combinations. But no two nematodes or white grub species had the same pattern for these observations for all white grub or nematode species, respectively. Mortality, establishment, and speed of kill followed a similar pattern for H. zealandica, S. glaseri, and S. scarabaei, but there was no clear relationship for H. bacteriophora. Significant nematode establishment was only observed after at least 48 h exposure in most nematode-white grub combinations. Faster establishment was observed only for H. zealandica in A. orientalis and R. majalis (after 24 h) and for S. scarabaei in P. japonica and R. majalis (after 12 h). Nematode establishment after 72 h in the different scarab species was generally low for S. glaseri (

Entomopathogenic nematodes are used for inundative biological control of insect pests but also have potential for use in augmentative and/or inoculative biological control. Numerous new species are being described but generally, little information is provided on their ecology. Moreover, many species have been described previously without subsequent ecological information. This paper presents a number of simple experiments examining a nematode's physiological host range, host-foraging behavior, thermal activity range, and soil moisture activity range that provides the basic information necessary for further studies of a new species as a biological control agent. We suggest that any taxonomic description of new species should be combined with such an ecological study. This kind of information should also be generated for species in which no ecological studies have been done. In our study, we found that the entomopathogenic nematode Steinernema rarum which was described in 1986 infected a wide range of arthropod species under laboratory conditions but appeared to be best adapted to lepidopteran larvae as hosts. It used an intermediate foraging strategy infecting both mobile hosts on the soil surface and sedentary hosts below the soil surface efficiently. The range of soil moisture at which S. rarum was effective suggested an adaptation to the fluctuating soil moisture in the upper soil layers. Its temperature optimum of 25 degreesC corresponded well to the warm temperate climate of its area of origin in Argentina. The optimal storage temperature was around 15 degreesC. Our study suggests that S. rarum is well adapted to lepidopteran hosts with larvae spending part of their live on the soil surface and/or that pupate in the soil. Copyright 1999 Academic Press.